The present invention relates to a drive unit for a motor vehicle, comprising a drive engine with an output shaft, a transmission, which has an input shaft and at least one intermediate shaft with an intermediate shaft brake, a friction clutch with a first friction element connected to the output shaft and a second friction element connected to the input shaft, a clutch operating mechanism for engaging and disengaging the clutch, an electronic control unit for controlling the operating mechanism in dependence on various control parameters, and a position detector, which detects the position of the clutch operating mechanism and transmits a position-dependent signal to the control unit.
The invention also relates to a method for determining the torque-transmitting characteristic of a friction clutch which has a first friction element coupled to an output shaft from a drive engine and a second friction element coupled to an input shaft to a vehicle transmission, which transmission has an intermediate shaft with an intermediate shaft brake and a clutch operating mechanism, controlled by an electronic control unit, for regulating the engagement and disengagement of the clutch, the starting point for which method is that the engine is started, the transmission is in neutral position and the clutch is disconnected.
At present, a variety of methods are used to identify a so-called disconnection position of vehicle clutches. The clutch suppliers define where (expressed in terms of position of the clutch operating mechanism) the clutch is fully disconnected and does not transmit any torque. This position is defined with a safety margin, so that all clutches of a certain model will always with great certainty be fully disconnected, so that there is no risk of the clutch lying and slipping in the disconnected position and being subjected to unnecessary wear. For a typical clutch, this means that the clutch is controlled to a position lying unnecessarily far beyond the position in which the clutch torque is zero. This means, in turn, that the engagement process takes longer than if the process could be initiated from the position in which the torque becomes zero upon disconnection.
Correspondingly, there are a number of methods for determining the so-called traction position of the clutch, i. e. the position in which the clutch transmits a certain low torque. The traction position constitutes a reference point for controlling the movements of automated clutches where the gas pedal of the vehicle, for example during a starting process, acts as an pedal, which, when gas is applied from idle, increases the engagement of the clutch from the traction position to full engagement.
In a method for calculating the torque transmission of a vehicle clutch as a function of the position of the operating mechanism, which method is known by virtue of SE-A-517 743, the operating mechanism is put into a number of mutually different positions. The time from a predetermined low rev speed at the input shaft to a predetermined higher rev speed is measured for all chosen positions and the torques in the various engagement positions are calculated from the moment of inertia and the angular acceleration of the masses rotating in the neutral position of the transmission. A torque curve as a function of the position of the clutch operating mechanism can now be interpolated and/or extrapolated on the basis of the calculated torque values and, from this, the disconnection position and traction position can be determined with relatively high precision.
It is desirable to produce a drive unit of the type stated in the introduction, which, with simple means and absolute precision, can identify the disconnection position and traction position of a vehicle clutch.
This is achieved according to the invention by the fact that a torque transmitter is provided, which detects the torque at the input shaft and transmits a torque-dependent signal to the control unit, and that the control unit is arranged so as, on predetermined occasions, when the transmission is in the neutral position and the intermediate shaft brake is applied, gradually to alter the degree of engagement of the clutch and to register and store, as a function of the position of the clutch operating mechanism, the torque measured by the torque transmitter.
In a preferred embodiment, the control unit is arranged so as, when the clutch is disconnected and the intermediate shaft brake is applied, gradually to engage the clutch and to register and store, as a function of the position of the clutch operating mechanism, information on the torque measured by the torque transmitter.
It is desirable to produce a secure and simple method for determining, in a drive unit having a transmission with intermediate shaft brake, the torque-transmitting characteristic of the clutch of the drive unit.
This is achieved according to the invention by the fact that, with the engine started and the transmission in neutral position;                the intermediate shaft brake is applied,        the degree of engagement of the clutch is gradually altered,        input torque to the transmission is detected, during the alteration, by means of a torque transmitter,        the position of the clutch operating mechanism is detected, during the alteration, by means of a position detector, and        the transmitted torque is stored in the control unit as a function of the position of the clutch mechanism.        
The control unit can be programmed to conduct the described procedure for determining the torque characteristic of the clutch at certain set intervals or each time the engine is started. In this way, full control over changes in torque characteristic owing to plate wear, temperature changes, etc. is obtained, so that the clutch operating mechanism, upon disconnection, is always put into the position in which the torque becomes zero upon disconnection and, upon engagement, is put into the correct traction position. The procedure is concluded, at the latest, when the intermediate shaft brake is no longer capable of offering resistance to the clutch.
The invention also relates to a computer program and computer program product which comprise a program code according to the above-stated procedural steps.